Heat-exchange tubing and method of making it

ABSTRACT

Heat-exchange tubing with a peripheral wall of oblong crosssection, and inner fins on the wall of which the fins on either of two opposite flat wall sections extend with their tips at least to the level of the tips of the fins on the other flat wall section, and a method of forming the tubing from a round innerfin tube blank, involving partially flattening the round blank into the tubing with its peripheral wall of oblong crosssection.

United States Patent 1151 3,662,582

French 1 May 16, 1972 541 HEAT-EXCHANGE TUBING AND 1,177,320 3/1916Grabowskymn ..29/157.3

METHOD OF MAKING IT [72] Inventor: Fred W. French, Morris, Conn. PnmaryEmmmer loweu Larson Attorney-Walter Spruegel [73] Assignee: NorandaMetal Industries Inc.

22 Filed: May 18, 1970 ABSTRACT [21] Appl. No.: 38,132 Heat-exchangetubing with a peripheral wall of oblong crosssection, and inner fins onthe wall of which the fins on either of two opposite flat wall sectionsextend with their tips at least to {2?}33311111111111.............Z'.'.Z'Z??Ti3????33532932 ehe heveh ef he theeh ehe en the ehhee hee [58] Field of Search ..72/256, 367; 29/ 157.3 A,157.3 AH and a method of forming the tubing from a round inner-fin 291573 1 13/1 1 A 11 1 5/177 179 tube blank, involving partiallyflattening the round blank into the tubing with its peripheral wall ofoblong cross-section.

[56] References Cited UNITED STATES PATENTS 3,422,518 l/l969 French..29/l57.3

8 Claims, 23 Drawing Figures PATENTEDMAY 16 I572 3.562.582

SHEET 1 [IF 2 INVENTOR ATI' RNEY PATENTEDMAY 16 I972 3,662,582

sum 2 OF 2 10 Q fig Q 5 10 HEAT-EXCHANGE TUBING AND METHOD OF MAKING ITThis invention relates to heat-exchange tubing in general, and to finnedheat-exchange tubing in particular.

The type of heat-exchange tubing with which the present invention isconcerned is provided with inwardly extending fins, or so-called innerfins, on its peripheral wall. Tubing of this type is well known for itsheat-exchange properties which vary from good to excellent, depending onthe inner-fin pattern and size, the particular heat-exchangeapplication, and other factors. However, even this type of tubing doesnot lend itself to certain exacting heat-exchange requirements forvarious applications. There are several reasons for this, and chiefamong them is that heat-exchange of the fins and also peripheral wall ofsuch tubing with fluid passing through the latter is inadequate forcertain purposes regardless of the height and number of the fins.

It is the primary object of the present invention to provideheat-exchange tubing of this type which meets many exactingheat-exchange requirements that cannot be met by the aforementionedknown inner-fin tubing.

It is another object of the present invention to provide heatexchangetubing of this type of which the peripheral wall and the inner fins arearranged to divide the entire interior of the tubing into individualflow channels of a number, depth and width to best meet specifiedheat-exchange requirements as well as other requirements, such as aspecified volumetric flow rate of a fluid through the tubing, or to keeppressure drop of the fluid in the tubing within specified limits, forexample.

It is a further object of the present invention to provide heat-exchangetubing of this type the interior of which is divided into flow channelsfor meeting various specific, including heat-exchange, requirements asaforementioned, by making the peripheral wall generally oblong incross-section, with the same providing two opposite flat wall sectionsand opposite return wall sections which join the flat wall sections, andthe fins on either flat wall section extend with their tips at least tothe level of the tips of the inner fins on the other flat wall section.It is thus within the wide parameters of oblong cross-section of theperipheral wall and the number, height and spacing of the fins, that agreat variety of heat-exchange tubing for many different applicationsmay be fashioned.

Another object of the present invention is to devise a method'of formingheat-exchange tubing d this type, which comprises providing a roundinner-fin tube blank, and partially flattening the blank into theaforementioned oblong cross-section of its peripheral wall at which thefins on either one of the then opposite flat wall sections extend withtheir tips at least to the level of the tips of the fins on the otherflat wall section. In thus forming the heat-exchange tubing, which mayaptly be termed flat tubing, the number, height, spacing and directionof the fins therein may be selected from the wide variety of finpatterns and sizes which may readily be formed in round tubular blanksaccording to different known methods, but which could hardly, and neverpractically, be formed in flat tubing. Moreover, extreme simplicitycharacterizes the reformation of a round inner-fin tube blank into flattubing of this kind in accordance with the present method, as by passingthe round blank between rotary companion rolls or drawing the samethrough a die, all in a single pass, for example. Moreover, reformationin this fashion of a round tube blank particularly with helical innerfins into flat tubing of this type brings the fins into an entirely newand extremely effective cooperative relation, in that the then-straightfins on the respective flat wall sections abut and are inclined to andcross each other, with the result that these fins sharply divide anddivert into different directions at each cross-section of the tubingmuch of the fluid flowing through the entirely finned passage in thetubing.

A further object of the present invention is to provide flat tubing ofthis type whose heat-exchange with a fluid passing therethrough isfurther enhanced, in that in the aforementioned partial flattening of around inner-fin tube blank into the flat tubing, the flat opposite wallsections are spaced apart a distance at which the fins on either flatwall section extend with their tips beyond the level of the tips of thefins on the opposite flat wall section but remain spaced from thelatter. With this arrangement, the path of fluid through the tubing iseven more tortuous past the fins therein especially where the fins onthe opposite flat wall sections cross each other, involving additionaldiversion of fluid within the channels between successive fins over thetips of opposite fins projecting within the confines of the channels.Further, where the fins on the op posite flat wall sections are inclinedto and cross each other, the fins will over the extent of theirinterpress at their crossings readily give way in denting and thereinterlock without distorting the fin pattern.

It is another object of the present invention to provide heatexchangetubing of this type which, if desired, may have graduated heat-exchangeproperties over different lengths or from one end to the other end, bysimply partially flattening a round inner-fin tube blank to differentextents so that the sets of inner fins on the respective flat oppositewall sections vary in their relative projection from level at their tipsto interprojectron.

It is a further object of the present invention to form heatexchangetubing of this type according to the aforementioned method, and which issubsequently further deformed in crosssectionally or longitudinallycurved fashion, thereby to reenforce the tubing against spread-apart oftheir opposite flat wall sections under pressure from fluid passingthrough the tubing.

Another object of the present invention is to provide heatexchangetubing of this type which for any, and even exceptional, length and,hence, heat-exchange capacity, may be of very condensed lengthwiseconstruction, by lengthwise bending the tubing into more or less closelyadjacent, successive helical turns, as around a cylindrical mandrel, forinstance.

It is another object of the present invention to provide heatexchangetubing of this type which is formed, according to the aforementionedmethod, from a round tube blank with inner and outer fins, so that thetubing has by virtue of the additional outer fins further enhancedheat-exchange properties. The outer fins on the round tube blank maylongitudinally extend parallel to, or helically about, the tube axis,with neither axial nor helical outer fins interfering with orderlypartial flattening of the blank on providing for suitable clearance ofthe outer fins in the blank-flattening tooling.

Further objects and advantages will appear to those skilled in the artfrom the following, considered in conjunction with the accompanyingdrawings.

In the accompanying drawings, in which certain modes of carrying out thepresent invention are shown for illustrative purposes: 5

FIG. 1 is a plan view of heat-exchange tubing embodying the invention; 7i

FIGS. 2 and 3 are sections through the tubing taken on the lines 2-2 and3-3, respectively, in FIGS. 1 and 2;

FIG. 4 is a cross-section through a round inner-fin tube blank fromwhich the tubing of FIGS. 1 to 3 is fashioned;

FIG. 5 is a section through the tube blank on the line 5-5 in FIG. 4;

FIG. 6 is a cross-section through heat-exchange tubing embodying theinvention in a modified manner;

FIG. 7 is a section through the modified tubing substantially along theline 7-7 in FIG. 6;

FIG. 8 is an enlarged section through part of the modified tubingsubstantially along the line 8-8 of FIG. 7;

FIG. 9 is a cross-section through heat-exchange tubing embodying theinvention in another modified manner;

FIG. 10 is a section through the modified tubing of FIG. 9 along theline 10-10 thereof;

FIG. 1 l is a cross-section through heat-exchange tubing embodying theinvention in a further modified manner;

FIG. 12 demonstrates a step in the formation of heatexchange tubingaccording to a method which also embodies the invention;

FIG. 13 demonstrates a modified step in the formation of heat-exchangetubing according to a method of the invention;

FIG. 14 is a cross-section through heat-exchange tubing of still anothermodification;

FIG. 15 is a side view, partly in section, of a heat exchanger embodyingthe featured tubing;

FIG. 16 is a section through the heat-exchanger along the line 16 -16 inFIG. 15;

FIG. 17 is a side view, partly in section, of a modified heatexchangerembodying the featured tubing;

FIG. 18 is a view of the featured heat exchange tubing with alongitudinal twist;

FIG. 19 is a section through the featured heat-exchange tub-' ing whichis also cross-sectionally curved;

FIG. 20 is a perspective view of the featured heat-exchange tubing whichis also bent longitudinally into successive helical turns;

FIG. 21 is a cross-section through heat-exchange tubing embodying theinvention in a further modified manner;

FIG. 22 is a cross-section through a round finned tube blank from whichthe heat-exchange tubing of FIG. 21 is fashioned; and

FIG. 23 is a cross-section through heat-exchange tubing embodying theinvention in a still further modified manner.

Referring to the drawings, and more particularly to FIGS. 1 to 3thereof, the reference numeral designates heatexchange tubing having aperipheral metal wall 12 of oblong cross-section and a multitude ofmetal fins 14 with tips 16. The peripheral wall 12 provides two flatopposite, and preferably parallel, wall sections 18, and opposite returnwall sections 20 which join the flat wall sections 18, with the flatwall sections 18 constituting in this instance a far predominant part ofthe wall 12. The fins 14, which project inwardly from the wall 12 andare preferably formed integrally therewith, are of the same height whichis such that the fins on either flat wall section 18 extend with theirtips 16 to the level of the tips of the fins on the opposite flat wallsection (FIG. 2), sothat the entire interior of the flat tubing iswithin reach of the fins. Successive fins 14 on the wall 12 arepreferably equally spaced, and the fins on either flat wall section 18extend parallel to each other and at an inclination to the longitudinalaxis x of the tubing, with the fins on the respective wall sections 18being also inclined to and crossing each other (FIG. 3).

With the interior of the flat tubing being within full reach of the fins14, the entire passage through the tubing is divided into individualflow channels 22, which makes for good heatexchange between a fluidpassing through the tubing and the fins 14 as well as peripheral wall 12of the tubing. Heatexchange between such fluid and the fins andperipheral wall of the tubing is even enhanced by the inclination toeach other of the channels 22 on the opposite flat wall sections 18(FIG. 3), in that they sharply divide and divert into different channelsmuch of the fluid passing therein at each cross-section of the tubing.

The flat metal tubing 10 is advantageously formed from a round inner-fintube blank 24 (FIGS. 4 and 5) in accordance with an exceedingly simplemethod. For reasons more fully apparent hereinafter, the peripheral wallof the blank 24 is of the same thickness and peripheral extent as thewall 12 of the flat tubing 10, and the fins of the blank are of the sameheight and thickness, and also spaced, as the fins 14 of the tubing,wherefore the peripheral wall and fins of the blank are appropriatelydesignated by the reference numerals 12 and 14, respectively, i.e., thesame as their counterparts of the flat tubing. Further, the fins 14 onthe round wall 12 of the blank 24 extend longitudinally helically at thesame helix angle throughout (FIG. 5).

The inner-fin tube blank 24 itself may be formed in any known manner,including brazing or otherwise joining inserted fins to the round wallof the blank, but preferably by displacement, according to differentknown methods, of metal from the wall of the blank into grooves on amandrel therein to form the fins 14 integral with the wall. One suchmethod is disclosed in my prior US. Pat. No. 3,422,518, dated Jan. 211969, with this method involving externally swaging a cylindrical tubeblank against a grooved mandrel therein in a single pass of the blankover and beyond the mandrel, whereby metal from the blank wall isdisplaced into the mandrel grooves to form the fins. This method ispreferred, not only because the same is highly efficient and readilylends itself to the formation of an inner-fin tube blank of most anydesired fin pattern and size, but also because the swaging of the blankover the mandrel entails quite extensive elongation of the blank. Suchextensive elongation of the blank and the formation of the finsexclusively by metal from the blank wall entail a considerable reductionof the wall thickness of the finished inner-fin tube blank, which ishighly advantageous in point of heat-exchange of the tube wall, andhence also entire tube, with a surrounding temperature-modifying medium,such as a coolant, for example.

The method of forming the inner-fin tube blank 24 into the flat tubing10 simply involves partially flattening the blank to form oppositeperipheral wall portions thereof into the flat parallel wall sections18, which concludes the formation of the flat tubing 10. Such partialflattening of the round blank 24 may be achieved in any suitable manner,as by passing the blank between rotary companion rolls 30 and 32 in thedirection of the arrow 34 (FIG. 12), or by drawing the blank through adie 36 in the direction of the arrow 38 (FIG. l3).

It follows from the preceding that the peripheral wall 12 of the flattubing 10 is indeed the same wall 12 of the blank 24 which remains ofthe same thickness and peripheral extent. It

is now also apparent that the fins 14 of the blank 24 and of the flattubing 10 are indeed the same and retain their height and thickness aswell as their spacing from each other. Further, in the course ofpartially flattening the round blank 24, the helically extending fins 14will over theextent of the flat wall sections 18 of the tubing beextended into straight disposition (FIG. 3). v

To bring the fins 14 for all practical purposes within full reach of theinterior of the flat tubing 10, the fins in the round tube blank mustobviously be spaced some distance from the axis of the blank. In thisconnection, it has been found that for a given inside diameter of theblank, the fin height may vary widely from less than the thickness ofthe peripheral blank wall to many times such wall thickness, with thefins of any height within this wide range being adequately spaced fromthe axis of the blank for its formation into flat tubing in which thefins are within full reach of the interior of the tubing. Within thiswide range of fin height, and with available round inner-fin tube blanksof many different fin patterns and sizes, it is possible to obtainwidely different flat inner-fin tubing which not only has goodheat-exchange properties, but also meets other requirements, such as aspecified volumetric flow rate of fluid through the tubing, or to keeppressure drop of passing fluid in the tubing within prescribed limits,for example. Thus, the number of fins, also their height within theabove wide range, and the peripheral extent of the wall, of flat tubingmay vary widely to meet many different, including heat-exchange,requirements. Insofar as the height of the fins is concerned, the sameis for many, but not all, applications greater than the thickness of theperipheral wall of the tubing.

Reference is now had to FIGS. 6 and 7 which show flat inner-fin tubing10a that basically differs from the described tubing 10 in that the fins14a on either flat wall section extend with their tips 16a beyond thelevel of the tips of the fins on the opposite flat wall section butremain spaced from the latter. The flat tubing 10a may otherwise be likethe tubing 10 and, hence, formed from the same round inner-fin tubeblank 24 (FIGS. 4 and 5), with the tubing 104 being formed by the samemethod as the tube 10, except that the round blank is partiallyflattened to an extent at which the fins on the opposite flat wallsections interproject. In thus partially flattening the round blank, thefins 14a on the opposite flat wall sections 18a are at, and over theextent of, their crossings 40 interpressed and thereby interlocked dueto mutual denting of the fins thereat as at 42 (FIG. 8). Thus, due tothe mutual denting of the fins at their crossings in consequence ofpartially flattening the round blank to the extent of part-wayinterprojecting the fins on the opposite flat wall sections, the finpattern as such remains intact and is not distorted (FIG. 7). Owing tothe part-way interprojection of the fins in this tubing, the fluid paththerethrough is quite tortuous in any event, and may even varyconsiderably with difi'erent degrees of interprojection of the fins.Different interprojection of the fins is thus another tool towardachieving good heat-exchange and meeting other widely varyingrequirements, such as volumetric flow rate of a fluid passing throughthe tubing, or to keep pressure drop of the passing fluid withinprescribed limits.

Reference is now had to FIGS. 9 and 10 which show flat heat-exchangetubing 10!: that is formed from a round innerfin tube blank (not shown)in which the fins extend parallel to the axis of the blank. Thus, inpartially flattening the round blank in accordance with the presentmethod, all the fins 14b in the flat tubing extend parallel to thelongitudinal axis xb. In this exemplary flat tubing, the fins 14b on theopposite flat wall sections 18b interproject to some extent, though itis entirely obvious that by difierent partial flattening of the blankthe fins on the flat opposite wall sections 18b may interproject to adifferent extent, or the tips of the fins on either flat wall section18b may with their tips extend to the level of the tips of the fins onthe other flat wall section 18b.

In the case of flat tubing in which the fins extend parallel to thelongitudinal axis of the tubing, as in FIGS. 9 and 10, it is alsofeasible partially to flatten the round inner-fin tube blank to theextent where the fins on either flat wall section extend with their tipsto the opposite flat wall section, with such heatexchange tubing 10cbeing shown in FIG. 11. In this tubing 10c, successive fins 14c dividethe interior of the tubing into flow channels 22c which, in contrast tothose in the described tubing 10, 10a and 10b, are closed to each other.

In the described fiat heat-exchange tubing 10 to 10, the two oppositeflat wall sections constitute the predominant part of the peripheralwall of the tubing. While this is preferred for exacting heat-exchangeand also other requirements of many applications, such as cooling thetransmission oil of automotive vehicles, just to mention one suchapplication, the advantages of having the fins within full reach of theinterior of flat tubing are secured even where the two flat oppositewall sections do not constitute a predominant part, or even constituteless than one-half, of the peripheral wall of the tubing. Thus, FIG. 14shows flat heat-exchange tubing 10d of which the flat opposite wallsections 18d constitute less than one-half of the peripheral wall 12d ofthe tubing, with the round innerfin tube blank (not shown) from whichthe tubing is fashioned being, in accordance with the present method,partially flattened to an exemplary extent at which the fins 14d oneither flat wall section 18d extend with their tips to the level of thetips of the fins on the other flat wall section 18d. Further, theindicated fin height for the also indicated peripheral extent andthickness of the wall of the tubing obviously falls within theaforementioned fin-height range within which the fins in flat tubing arebrought within full reach of the interior of the tubing.

Reference is now had to FIGS. 15 and 16 which show a heat-exchange unit50 using a length of piece 52 of the featured flat inner-fin tubing, forexample a piece of the flat tube 10a of FIGS. 6 and 7. The opposite ends54 and 56 of the tube piece 52 are in communication with the interior ofcasings 58 and 60, with the tube ends 54 and 56 being fitted in, andconveniently brazed to, slots 62 in the respective casings 58 and 60.The casings 58 and 60 have tapped holes 64 and 66 for connection withconduits through which to lead a fluid, liquid or gas, to and from theunit 50 for temperature modification, such as cooling, for instance.

While in the described heat-exchange unit 50 the end casings 58 and 60and their slots 62 are rectangular in' section (FIG. 16), FIG. 17 showsa heat-exchange unit 70 ofwhich the end casings 72 and 74 are circularin section. To this end, the length or piece 76 of featured inner-fintubing is, in its formation from a round inner-fin tube blank, partiallyflattened only over its longitudinal extent I so that opposite endlengths 78 and of the tubing remain cylindrical, and these cylindricalend lengths 78 and 80 are connected with the casings 72 and 74.

In many heat-exchange applications, the fluid passing through thefeatured flat inner-fin tubing is under operating pressure which may besufiiciently high to open" the tubing by forcing the opposite flatsections of the peripheral wall more or less apart, such as the flatwall sections 18 to 18 of the described tubing 10 to 10, for example,and thereby greatly reducing the heat-exchange capacity of the tubing,if not rendering the tubing unfit for further use in a particularheat-exchange application. Opening of the tubing in this fashion andfrom this cause is in many cases prevented by additionally curving thesame longitudinally, or transversely, or both, and thereby reenforcingthe tubing against such opening. Thus, a length of the featured flatinner-fin tubing may be twisted about its longitudinal axis x (FIG. 18),whereby the tubing becomes curved, longitudinally as well astransversely, over its lengthwise extent, and is thereby reenforcedagainst opening under internal pressure. The tube length 90 may betwisted by forcing the same through a correspondingly twisting openingin a die 92.

FIG. 19 shows a piece 94 of the featured flat inner-fin tubing which istransversely curved for reenforcement against opening under internalpressure. The initially flat tube piece 94 may to this end be drawnthrough a die 96 with an opening of the outline of the curved tubing.

FIG. 20 shows a piece 98 of the featured flat inner-fin tubing which islongitudinally curved for reenforcement against opening under internalpressure. This is achieved in this instance by bending the flat tubingaround a cylindrical arbor 100. The exemplary tube piece 98 is of quiteextensive length with correspondingly large heat-exchange capacity, andin order greatly to reduce the lengthwise expanse of the longitudinallycurved tubing, the tubing is bent around the arbor 100 in successive andmore or less closely adjacent helical turns 102.

While the flat heat-exchange tubing described so far has onlyinner-fins, such flat tubing may have both, inner and outer fins. Thus,FIG. 21 shows flat heat-exchange tubing 104 which has inner and outerfins 106 and 108. The tubing 104 is, in accordance with the presentmethod, formed from the round inner-and outer-fin tube blank 110 (FIG.22). The outer fins 108 extend in this instance parallel to the axis ofthe blank, but they may also extend helically, with the partialflattening of the round tube blank into the flat tubing being in eithercase entirely feasible on providing companion flattening rolls, forexample, with suitable slots for clearing the outer fins.

Reference is now had to FIG. 23 which shows flat heatexchange tubing 10ethat may be like the tubing 10 of FIG. 2, except that there isinterposed between the tips 162 of the fins Me on the opposite flat wallsections 18: a longitudinal strip 1 12 of any suitable brazing material.One of these strip materials, which is commercially available, is knownto the trade as SIL-FOS and manufactured by Handy and Harman. Thebrazing strip 112 is inserted in the course of flattening the initiallyround inner-fin tube blank into the flat tubing 10e, with the strip 112, which is shown of exaggerated thickness for claritys sake, beingengaged by the tips of the fins. The flat tubing 102 is then heated, asin a furnace 114, for example, to melt the brazing strip 112 and bracethe fins together at their crossing tips, with the excess brazingmaterial spreading over nearby portions of the fins. The tubing l0e,being thus brazed together at the crossing tips of the fins, will notopen under operational, including particularly high, internal fluidpressures. Brazing of flat tubing at the crossing tips of the fins isindicated where higher internal operational fluid pressures areinvolved, and especially for applications of such tubing which requirethat the same remains flat and is not to be curved for reenforcementagainst opening under internal fluid pressure. Of course, brazing offlat tubing in this manner applies as fully for tubing in which theinner-fins become interpressed in the course of flattening the initiallyround inner-fin tube blank into the flat tubing, as in FIG. 6, forexample.

lclaim:

1. Method of forming longitudinal heat-exchange tubing, which comprisesproviding a round metal tube blank having an axis, a peripheral wallabout said axis, and longitudinal inner-fins on said wall, with saidfins being of equal height and spaced from said axis; and forming theentire interior of the blank into individual flow channels of a depthwithin the height of the fins, by partially flattening the blank fromtwo opposite sides into oblong cross-section with two opposite flatparallel wall sections at a spacing at which the fins on one flat wallsection extend with their tips at least to the level of the tips of thefins on the other flat wall section.

2. Method of forming longitudinal heat-exchange tubing as in claim 1, inwhich the fins on the peripheral wall of the round tube blank extendhelically thereof at the same helix angle, whereby in said partialflattening of the blank the fins on said flat wall sections are extendedstraight and are inclined to and cross each other.

3. Method of forming longitudinal heat-exchange tubing as in claim 2, inwhich the blank is partially flattened until the fins on either flatwall section extend with their tips beyond the level of the tips of thefins on the other flat wall section but are spaced from the latter, andmetal of the fins on the flat wall sections, respectively, is displacedat their crossings for interpress of the fins at their crossings intointerlock with each other.

4. Method of fonning longitudinal heat-exchange tubing as in claim 1, inwhich the tubing is subsequently bent into curved extensionlongitudinally thereof.

5. Method of forming longitudinal. heat-exchange tubing as in claim 1,in which the flat wall sections are subsequently bent intotransversely-curved parallel disposition.

6. Method of fonning longitudinal heat-exchange tubing as in claim 1, inwhich the tubing is subsequently bent longitudinally into successivehelical turns.

7. Longitudinal heat-exchange tubing as in claim 2, in which the fins onthe respective flat wall sections are brazed together at theircrossings.

8. Method of forming longitudinal heat-exchange tubing as in claim 2,which further comprises, in the course of partially flattening the blankand before the fins on one flat wall section extend with their tips tothe level of the tips of the fins on the other flat wall section,inserting between the tips of the fins on said flat wall sections,respectively, a longitudinal metal brazing strip; and subsequent to thepartial flattening of the blank heating the tubing to melt said stripand thereby braze the fins together at their crossings.

1. Method of forming longitudinal heat-exchange tubing, which comprisesproviding a round metal tube blank having an axis, a peripheral wallabout said axis, and longitudinal inner-fins on said wall, with saidfins being of equal height and spaced from said axis; and forming theentire interior of the blank into individual flow channels of a depthwithin the height of the fins, by partially flattening the blank fromtwo opposite sides into oblong cross-section with two opposite flatparallel wall sections at a spacing at which the fins on one flat wallsection extend with their tips at least to the level of the tips of thefins on the other flat wall section.
 2. Method of forming longitudinalheat-exchange tubing as in claim 1, in which the fins on the peripheralwall of the round tube blank extend helically thereof at the same helixangle, whereby in saId partial flattening of the blank the fins on saidflat wall sections are extended straight and are inclined to and crosseach other.
 3. Method of forming longitudinal heat-exchange tubing as inclaim 2, in which the blank is partially flattened until the fins oneither flat wall section extend with their tips beyond the level of thetips of the fins on the other flat wall section but are spaced from thelatter, and metal of the fins on the flat wall sections, respectively,is displaced at their crossings for interpress of the fins at theircrossings into interlock with each other.
 4. Method of forminglongitudinal heat-exchange tubing as in claim 1, in which the tubing issubsequently bent into curved extension longitudinally thereof. 5.Method of forming longitudinal heat-exchange tubing as in claim 1, inwhich the flat wall sections are subsequently bent intotransversely-curved parallel disposition.
 6. Method of forminglongitudinal heat-exchange tubing as in claim 1, in which the tubing issubsequently bent longitudinally into successive helical turns. 7.Longitudinal heat-exchange tubing as in claim 2, in which the fins onthe respective flat wall sections are brazed together at theircrossings.
 8. Method of forming longitudinal heat-exchange tubing as inclaim 2, which further comprises, in the course of partially flatteningthe blank and before the fins on one flat wall section extend with theirtips to the level of the tips of the fins on the other flat wallsection, inserting between the tips of the fins on said flat wallsections, respectively, a longitudinal metal brazing strip; andsubsequent to the partial flattening of the blank heating the tubing tomelt said strip and thereby braze the fins together at their crossings.